The Segni (sometimes Segui) were a tribe living in Belgic Gaul when Julius Caesar's Roman forces entered the area in 57 BCE. They are known from his account of the Gallic War. They were one of a group of tribes listed by his local informants as the Germani of Belgian Gaul, along with the Eburones, Condrusi, Paemani (or Caemani), and Caeroesi (or Caeraesi). The Segni do not appear in the first listing of the Germani, which was a listing of Germani sending men to fight Caesar. But they appear in a later mention, after the defeat of the Eburones:

The Segui and Condrusi, of the nation and number of the Germans [Germani], and who are between the Eburones and the Treviri, sent embassadors to Caesar to entreat that he would not regard them in the number of his enemies, nor consider that the cause of all the Germans on this side the Rhine was one and the same; that they had formed no plans of war, and had sent no auxiliaries to Ambiorix. Caesar, having ascertained this fact by an examination of his prisoners, commanded that if any of the Eburones in their flight had repaired to them, they should be sent back to him; he assures them that if they did that, he will not injure their territories.

These tribes are referred to as the " Germani Cisrhenani", to distinguish them from Germani living on the east of the Rhine, outside of the Gaulish and Roman area. Whether they actually spoke a Germanic language or not, is still uncertain. The region was strongly influenced by Gaul, and many of the personal names and tribal names from these communities appear to be Celtic. But on the other hand it was claimed by Tacitus that these Germani were the original Germani, and that the term Germani as it came to be widely used was not the original meaning. He also said that the descendants of the original Germani in his time were the Tungri.

The general area of the Belgian Germani was between the Dijle (Dyle) and Rhine rivers, and north of Luxemburg and the southern parts of the Eifel. In modern terms this area includes eastern Belgium, the southeastern parts of the Netherlands, and a part of Germany on the west of the Rhine, but north of the Moselle valley.

The specific location of the Segni, as can be seen from the brief mention of Caesar, quoted above, was between the Eburones and the Treverii, somewhere in the region of the Ardennes. The Condrusi, mentioned as living in the same area and being part of the same embassy to Caesar, are thought to have lived in the Condroz region in the north of the Ardennes.

In the 19th century, it was sometimes claimed that the name of the Segni is preserved in a modern town of "Sinei or Signei", on the Meuse river, in the Belgian province of Namur.

It has also occasionally been claimed that the Segni appear as the " Sunuci" or "Sinuci" in later Roman records, such as the Naturalis Historia of Pliny the Elder. If so, then we have more records to refer to. Pliny described them between the Tungri and the Frisiavones. Tacitus, for example, also mentioned the Sunuci, as a people of this region during the Batavian revolt. They probably lived between the Tungri and the Ubii in Roman imperial times.

The Sunuci are thought to have lived in what is now the area of Germany where it touches eastern Belgium, and the southern Netherlands. One proposal would place the Sunuci in Kornelimünster in the region of modern Aachen.